Multi-cylinder internal combustion engines, particularly diesel engines for large tractor-trailer trucks, may include an exhaust-gas turbocharger. The turbocharger includes a turbine that drives a compressor via a shaft, which generates an increased intake air pressure in the intake duct during normal operation.
The turbine shaft is typically supported on two bearings within a central housing between the turbine and the compressor. Lubricating oil is supplied through a port in the central housing wall and split through oil channels to feed both bearings. Turbochargers commonly use crankcase oil to lubricate the rotating bearing interfaces as well as the thrust surfaces that limit axial excursions of the shaft and its turbine and compressor wheels.
Some examples of turbochargers and bearing lubrication systems can be found in U.S. Pat. Nos. 6,709,160; 4,902,144; 6,418,722 and 5,076,766, herein incorporated by reference.
In turbocharger systems, oil may leak across the turbo bearings into the compressor housing. The bearings support a rotatable shaft on which a turbine and compressor wheels are fixedly mounted. A turbocharger installed in an internal combustion engine is usually provided with a shaft seal arrangement for preventing lubricating oil supplied to the bearings from leaking into a compressor housing of the turbocharger. Oil may still leak into the compressor housing if the pressure in the compressor housing is lower than the pressure in the central housing.
Under motoring conditions, a vacuum force is generated on the outlet side of the compressor as a result of the continuous operation of the pistons and a decrease in the amount of exhaust gas available to operate the turbine-driven compressor. The pressure differential generated across the seals in the central housing causes oil in the housing to seep toward the compressor.
In some turbocharged internal combustion engines, an exhaust valve is disposed downstream of the turbine. Under certain operating conditions, such as to increase engine operating temperatures, or for engine braking, the exhaust valve is closed. When the exhaust valve is closed, a buildup of pressure occurs in the engine, which restricts rotation of the turbine. Under normal operating conditions, the turbine turns as a result of exhaust gas expanding as it moves across the turbine. When the valve is closed, a build up of pressure restricts expansion of the exhaust gas. As a result, the compressor is unable to compress sufficient air to the inlet air passage to maintain positive pressure in the inlet air passage. Under these circumstances, oil is prone to seeping into the compressor housing as a result of a pressure difference across the oil seal.
Normally, a positive air pressure inside the compressor prevents oil leakage into the compressor housing. However, under vacuum conditions generated under motoring conditions, or under operating conditions where the exhaust gas valve is closed, oil tends to seep into the compressor housing.
Various prior art patents, including U.S. Pat. Nos. 3,574,478 and 5,076,765, have attempted to address the issue of oil seepage under vacuum conditions.
The present inventors have recognized the need for a simple, efficient way of preventing oil from seeping into the compressor housing during motoring conditions, or when the exhaust valve is closed downstream of the turbocharger outlet.
The present inventors have recognized the need for a system which allows a turbocharged engine to act as a naturally aspirated engine at conditions when the engine is in a motoring condition.
The present inventors have recognized the need for a system which assists in maintaining the shaft seal arrangement while also increasing the air flow through the engine.